[yaspgrid,bugfix] Use canonical communicator type for non-MPI case

If people write programs the DUNE way then they will end up with
Dune::FakeMPIHelper::MPICommunicator as their communicator type.
Unfortunately Yaspgrid uses YaspGrid<dim, Coordinates> which is far
from being inituitive and broke vtktest. This patch resorts to using
the canonical choice, Dune::FakeMPIHelper::MPICommunicator, and fixes
the test.